Projection lighting fixtures concentrate light in a specific direction. These fixtures have been used for many years in various theater, television, architectural, and general illumination applications (e.g., overhead projection, spotlight illumination, semiconductor assembly, medical/scientific instrumentation, illumination of airport runways and high-rise buildings, etc.). Typically, these fixtures include an incandescent or a gas-discharge lamp mounted adjacent to a concave reflector, which reflects light through a lens assembly to project a narrow beam of light over considerable distance towards a target object.
In recent years, light-emitting diodes (LEDs) also have been used in some types of projection lighting fixtures. In particular, surface mount or chip-on-board assemblies of single or multiple LEDs have attracted attention in the industry for use in applications requiring high luminance combined with narrow-beam light generation (to provide tight focusing/low geometric spreading of illumination). A “chip-on-board” (COB) LED assembly refers generally to one or more semiconductor chips (or “dies”) in which one or more LED junctions are fabricated, wherein the chip(s) is/are mounted (e.g., adhered) directly to a printed circuit board (PCB). The chip(s) is/are then wire bonded to the PCB, after which a glob of epoxy or plastic may be used to cover the chip(s) and wire connections. One or more such LED assemblies, or “LED packages,” in turn may be mounted to a common mounting board or substrate of a lighting fixture.
For some narrow-beam applications involving LED chips or dies, optical elements may be used together with the LED chip-on-board assembly to facilitate focusing of the generated light to create a narrow-beam of collimated light. Collimated light is light whose rays are parallel and thus has a planar wavefront. Optical structures for collimating visible light, often referred to as “collimator lenses” or “collimators,” are known in the art. These structures capture and redirect light emitted by a light source to improve its directionality. One such collimator is a total internal reflection (“TIR”) collimator. A TIR collimator includes a reflective inner surface that is positioned to capture much of the light emitted by a light source subtended by the collimator. The reflective surface of conventional TIR collimators is typically conical, that is, derived from a parabolic, elliptical, or hyperbolic curve.
Referring to FIG. 1, a conventional TIR collimator 100 collects the light emitted by an LED light source 112 (which may include an LED chip-on-board assembly, or “LED package,” including one or more LED junctions) and directs the light so that it exits the collimator at a top portion 113. Some of the light travels from source 112 through a primary optic 114, into a first cavity 116, through a centrally-located lens 118, and out via a second cavity 120. The remainder of the light exits via a transparent surface 122 or a flange 124, which is used to retain collimator 100 in a holder (not shown). The light that does not pass through the central lens is incident on an inner sidewall 126 and is refracted as it passes from the air in the first cavity into the plastic material of the collimator. Thereafter, it is reflected at an inner reflective surface 129. The reflected light is refracted again as it travels from the plastic body of the collimator to the ambient air, at transparent surface 122. The reflective surface is conical, so that a cross-sectional profile of the collimator is parabolic at the reflective surface, as shown in FIG. 1.
In the collimator shown in FIG. 1, the reflection at reflective surface 129 occurs by total internal reflection, establishing constraints on the overall shape and curvature of the cross-sectional profile of the reflective surface. Due to the difference between the refractive index of collimator 100 and the refractive index of the ambient air, Snell's law applies and defines a critical angle for the angle of incidence, which is made by an incident light ray with respect to a normal to the reflective surface. That is, for incident angles above the critical angle, all of the light is reflected and none is transmitted through the reflective surface 129 or along the surface 129, thereby providing total internal reflection. For a plastic (refractive index of about 1.59)-air (refractive index of 1) interface, the critical angle is about 39 degrees. Thus, the reflective surface 129 is sloped to provide an angle of incidence for most of the light that is greater than about 39 degrees.
In theory, conventional collimators are capable of producing perfectly collimated light from an ideal point source at the focus. However, when these collimators are used in real-life applications with a light source of an appreciable surface area (such as an LED light source), the light is not completely collimated, but rather is directed into a diverging conic beam. For example, the light output from a conventional LED source (e.g., a COB LED assembly) may be emitted in a cone having a beam divergence of approximately 110 degrees (i.e., approximately 55 degrees to either side of a central axis in a direction of light propagation), and a collimator similar to that shown in FIG. 1 may redirect the generated light into a narrower cone-shaped beam having a divergence of approximately 10 degrees (i.e., approximately 5 degrees to either side of the central axis).
In some narrow-beam applications involving an LED package (e.g., a COB LED assembly including one or more junctions) and a corresponding collimator, the phenomenon of chip or die imaging may be problematic. In particular, with relatively small collimators used in conjunction with an LED package, the generally square or rectangular shape of the light emitting portion of an LED package (i.e., the arrangement of one or more chips in the package) may create a similar square or rectangular shape in the far field irradiance distribution pattern of the light projected from the collimator. In this manner, the square or rectangular shape of the light emitting portion of the LED package may be “imaged” in the far field due to collimation, which may result in undesirable irradiation non-uniformity in some circumstances. Moreover, with LED packages including multiple junctions that generate respective different wavelengths of light (“multi-color packages,” such as RGGB, BGGA and RGBW), the challenge of narrow-beam optical design is even greater, due to the color disparity within the package.